The present invention relates to a progressive power spectacle lens having both a distance portion and a near portion.
FIG. 9 schematically shows an example of a conventional progressive power spectacle lens having a distance portion 1 and a near portion 3. As shown in FIG. 9, in this example, the distance portion is located on an upper area, the near portion is located on the lower portion, and at an intermediate portion 2 therebetween, a power is progressively changes depending on a location between the distance portion and the near portion. Conventionally, both of a lens formed with a progressive power surface on an outer side surface (i.e., an object side surface: a front surface) and a lens formed with a progressive power surface on its inner surface (i.e., an eye side surface: a back surface) have been known. In such a progressive power lens, astigmatism AS is defined as follows:
AS=Dmaxxe2x88x92Dmin
where,
Dmax represents a maximum sectional surface power; and
Dmin represents a minimum sectional surface power.
In the spectacle lenses such as the progressive power spectacle lenses, it is desirable that the lens is as thin as possible, and that the lens has less astigmatism on a main meridian MMxe2x80x2 which is a virtual line (linear or curved line) passing the center of the lens and extending substantially in an up-and-down direction in FIG. 9. When a person wearing a spectacle sees an object at a long distance and an object at a short distance, a line of sight passes along the main meridian MMxe2x80x2. Preferably, there is no astigmatism along the main meridian MMxe2x80x2.
In the conventional progressive power lens, in order to suppress the astigmatism on the main meridian MMxe2x80x2, a relatively deep base curve is employed as a base curve of the progressive lens surface.
A point on the surface of the lens where the astigmatism is zero can be regarded as an infinitesimal spherical surface and is generally called an xe2x80x9cumbilical pointxe2x80x9d. A row of successive xe2x80x9cumbilical pointsxe2x80x9d is called an xe2x80x9cumbilical meridianxe2x80x9d, and, in the conventional progressive power lens, the main meridian MMxe2x80x2 is formed to be the umbilical meridian (i.e., there is no astigmatism on an any point on the main meridian MMxe2x80x2).
In order to fabricate a thin progressive power lens, a relatively shallow base curve is to be employed for the progressive surface. However, if the base curve is excessively shallow, and the shape thereof along the main meridian is formed to be the umbilical meridian, astigmatism, which cannot be negligible, is generated on the main meridian. Further, in such a configuration, an area within which a clear vision having less astigmatism (hereinafter referred to as a clear vision area) becomes very narrow.
For example, if a progressive power lens whose SPH (vertex power) is xe2x88x924.00D and addition power is 2.00D is fabricated as a lens having a base curve of 2.00D, which is relatively shallow for this addition power, and the shape of the progressive surface along the main meridian is the umbilical meridian, the obtained lens has a characteristic as shown in FIG. 10.
FIG. 10 shows a distribution of transmissive astigmatism of the above-described progressive power lens. As understood from FIG. 10, astigmatism is generated on the main meridian MMxe2x80x2, and a clear vision area where the astigmatism is 0.5D or lower exists only in a narrow area from the lower part of the distance portion to the near portion.
FIG. 11 shows a distribution of surface astigmatism of this conventional lens. As shown in FIG. 11., there is substantially no surface astigmatism on the main meridian MMxe2x80x2.
FIG. 12 shows (which is also understood from FIG. 11) that the surface astigmatism increases at a portion farther from the main meridian MMxe2x80x2 in the horizontal direction (X-axis direction).
FIG. 13 is a graph showing variation of a minimum sectional surface power direction along a line, Y=10, of the conventional progressive power lens. As shown in FIG. 13, the absolute value of the minimum sectional refracting power direction xcex8 (unit: degrees) representative of an inclination angle of a direction, in which the sectional surface power is minimum, with respect to the X-axis (i.e., a horizontal direction of the lens in an xe2x80x9cas-wornxe2x80x9d condition) exceeds 40 at most part. The sectional surface power is defined as follows:
D=1000(nxe2x80x2xe2x88x92n)C,
where,
D represents a sectional surface power;
nxe2x80x2 represents a refractive index of medium on a rear side of the surface;
n represents a refractive index of medium in front of the surface; and
c represents curvature of the sectional surface.
As above, when a shallow base curve is used, and the progressive power surface along the main meridian is formed to be umbilical meridian, the astigmatism resides on the main meridian, and the clear vision area becomes relatively narrow.
In order to deal with the above problem, there are lenses which are formed such that the shape along the main meridian is not a row of umbilical points. Examples of such progressive power lenses are disclosed in Japanese Patent Provisional Publications No. SHO59-58415, No. HEI 01-22172, No. HEI 08-136868 and published Japanese translations of PCT international publication for patent application No. HEI 04-500870.
If the progressive power lens is formed such that the main meridian is not a row of umbilical points as described in the above publications, even if a shallow base curve is employed, the astigmatism on the main meridian is well suppressed. However, in the above-described publications, only a structure in the vicinity of the main meridian is disclosed. Therefore, although the astigmatism on the main meridian is well suppressed, the other problem of a relatively narrow clear vision area in the distance portion cannot be solved.
The present invention is advantageous in that a relatively wide clear vision area can be provided in the distance portion even though a shallow base curve is used for the progressive power surface.
According to an embodiment of the invention, an improved progressive power spectacle lens is provided. The progressive power spectacle lens has a progressive power surface on at least one of a front surface and a back surface of the spectacle lens. The progressive power surface includes a distance portion corresponding to a long-distance view, a near portion corresponding to a short-distance view and an intermediate portion between the distance portion and the near portion, a refracting power gradually changes in the intermediate portion between the distance portion and the near portion.
In such a spectacle lens, a shape of the progressive power surface along a main meridian is formed not to be umbilical, and the distance portion is formed to include an area where quantity of surface astigmatism decreases from a position on the main meridian to a predetermined position farther from the main meridian in the horizontal direction, the surface astigmatism then increasing at positions farther, in the horizontal direction, from the predetermined position.
With the above configuration, a relatively thin progressive power spectacle lens having a relatively wide clear vision area in the distance portion can be provided.
optionally, a spherical power of the spectacle lens is negative.
Still optionally, for y satisfying 5xe2x89xa6y less than 20, a condition:
AS(0,y) greater than 0.2
is satisfied,
where AS(x, y) represents quantity (unit: diopter) of surface astigmatism at a position (x, y), X and y being values on X and Y coordinate axes, respectively, an origin of the XY coordinate system being a fitting point of the spectacle lens, and for a combination of x and y satisfying 5xe2x89xa6y less than 20 and 10 less than |x| less than 30, a condition:
AS(0,y)xe2x88x92AS(x,y) greater than 0.1 
is satisfied.
Further optionally, for y satisfying 5xe2x89xa6y less than 20, conditions:
AS(0,y) greater than 0.2,
AS(xc2x130,y) greater than 0.2,
20 less than |xcex8(xc2x115,y)| less than 40, and
0 less than |xcex8(xc2x125,y)| less than 20,
are satisfied, where xcex8(x, y) represents the minimum sectional surface power direction xcex8 at coordinates of x and y.